The proposed project of Drs. Eduardo Rosa-Molinar, the University of Puerto Rico-Rio Piedras (UPR-RP), and Scott E. Fraser of the California Institute of Technology (CalTech) will result in a greater understanding of the cellular interactions that guide the development of the spinal cord. In particular, the experiments will establish the phenotypes of the cells that contain the enzymes synthesizing and degrading retinoic acid (RA) and elucidate their role in guiding the genesis, migration, and fate of germinal cells in the ependyma of the embryonic spinal cord, specifically, secondary motor neurons. The experimental plan builds upon previous National Science Foundation funded work performed in the Rosa-Molinar laboratory. That work established both the RA metabolism and the functional neuroanatomy of the spinal cord in the developing and mature Western Mosquitofish, Gambusia affinis affinis (hereafter "mosquitofish"). The proposed project's test experiments on the interactions that guide the development and differentiation of the spinal cord will take advantage of some of the unique attributes of the mosquitofish. As described in detail elsewhere in this grant application, complementary work will be performed in the two laboratories.